Elsevier

Optical Materials

Volume 17, Issues 1–2, June–July 2001, Pages 19-25
Optical Materials

Lateral coupling – a material independent way to complex coupled DFB lasers

https://doi.org/10.1016/S0925-3467(01)00014-3Get rights and content

Abstract

We have developed a new technique for the fabrication of complex coupled distributed feedback laser (DFB) lasers. A metal grating patterned lateral to a narrow ridge waveguide laser couples to the evanescent part of the guided mode. No regrowth is required for the processing, which makes this approach applicable to all material systems. DFB lasers fabricated from InGaAs/AlGaAs quantum well and dot lasers, GaInNAs DFB lasers emitting at 1.3 μm and long wavelength lasers at 2 μm based on InGaSbAs/AlGaSbAs structures are described. The lasers show low thresholds, good efficiencies and a high sidemode suppression ratio.

Introduction

High performance single mode lasers are key devices for optical communication, spectroscopy and sensing applications. A widely employed structure is the distributed feedback laser (DFB), where a grating structure is incorporated into the laser waveguide. The emission wavelength of this device is determined by the effective refractive index of the waveguide and the grating period. There are two types of DFB lasers: index and gain coupled DFBs. In the former case, only the real part of the refractive index is modulated, a gain coupled laser also has a modulation of the imaginary part. In some devices a combination of gain and index coupling is realized, leading to complex coupled lasers. The standard fabrication for DFB lasers utilizes at least two growth steps. After the expitaxy of the lower cladding and the waveguide, the grating is etched into the waveguide or the active region. A second-growth is then needed to complete the laser structure.

Index coupled lasers pose less stringent requirements on the expitaxy and are therefore the commonly employed devices. However, due to the existence of two degenerate modes on both sides of the stopband, the single mode yield for a given target wavelength is quite low. Index coupled DFB lasers are sensitive to backreflected light, so an optical isolator is required in system applications. The introduction of a modulation of gain in DFB lasers strongly increases the single mode yield and the overall laser performance [1], [2]. While becoming an established technique for InP based devices [3], [4], the fabrication of gain coupled lasers in other material system has been hampered by the difficulties of the regrowth step. The regrowth interface is close or in the active region, so sophisticated cleaning procedures have to be applied in order to ensure a good interface quality. In some cases, e.g., for layers with a high content of reactive aluminum, regrowth might not be possible at all.

In order to avoid the overgrowth step, DFB lasers with lateral coupling [5] or coupling to a grating on top of the waveguide have been studied [6]. The former approach uses an etched grating lateral to a small ridge, the field leaking out of the waveguide couples to the grating. This eliminates the overgrowth, but the lasers are only index coupled. Putting a metal grating on top of a waveguide with a thin cladding layer results in gain coupling, the reported efficiencies however were quite low.

Our approach for the fabrication of high performance distributed feedback lasers is based on lateral metal gratings deposited on both sides of a narrow ridge waveguide. A schematic of a laterally coupled laser is shown in Fig. 1. The metal gratings provide a modulation of the gain without the need of overgrowth steps. Since only the evanescent part of the mode interacts with the grating, the confinement factor of the grating is around 10−4, two orders of magnitude lower than for an overgrown grating incorporated into the waveguide. This is compensated by the large imaginary part of the refractive index of the metal. We use Cr as grating metal, which has an absorption coefficient of 2.8×105cm−1 at a wavelength of 1 μm. The resulting gain coupling coefficients are around 5–20cm−1 in these devices, which is more than sufficient for stable single mode operation. A theoretical analysis shows that the additional losses introduced by the metal grating lead only to a slight penalty in threshold current and efficiency [7]. The nodes of the DFB mode are in phase with the absorption grating, resulting in small damping of the DFB mode.

Section snippets

Fabrication

Before processing, the complete laser structures are first characterized by fabrication of broad area lasers, allowing an evaluation of the layer quality. The main difference in the fabrication flow for different material systems is the definition of the ridge waveguide. Laser ridges with widths between 2 and 3 μm are defined by photolithography and a Ti/Ni mask with a total thickness of 150 nm is lifted off. Depending on the composition of the cladding layer, the ridge is either etched with an

InGaAs/AlGaAs quantum well lasers

MBE grown layers with a single InGaAs quantum well embedded in a GaAs/AlGaAs GRINSCH structure were used for these lasers [9]. Fig. 3(a) and (b) displays a typical output power characteristic and spectrum of a laterally coupled InGaAs/AlGaAs DFB laser. The ridge was 2.5 μm wide and 600 μm long. A threshold current of 9 mA is obtained. The differential quantum efficiencies of the devices are around 0.4 W/A and comparable to those of reference ridge waveguide lasers [10]. A high sidemode

Conclusion

To summarize our results, we have presented a novel approach for the fabrication of complex coupled DFB lasers. Due to the absence of any regrowth steps, the technique can be applied to all material systems. Only a small penalty for the threshold and output efficiency is introduced by the metal absorption grating, making the fabrication of high performance devices possible. The application of lateral coupling to new material systems such as quantum dot lasers, lasers with GaInAsN active layers

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